Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
in-stĭpŭlor, 1, v. dep., to bargain, stipulate for (Plautinian): dolo malo, Plaut. Rud. 5, 3, 25 (but id. Ps. 4, 6, 7, est stipulatus, Ritschl).